Current Projects

Projects:

Our work on the enteric nervous system spans stem cell biology, regenerative medicine, developmental biology, neurodegenerative disease, genetics, epidemiology, and gene-environment interactions. We are also actively pursuing new research avenues as we develop translational projects related to intestinal motility disorders.

Introduction to the Enteric Nervous System (ENS): The ENS is a beautiful and complex network of neurons and supporting glial cells that is distributed along the length of the bowel in two interconnected networks called the myenteric and submucosal plexus. These neurons control intestinal motility, respond to sensory stimuli from the bowel wall and bowel lumen, regulate intestinal epithelial secretion and control intestinal blood flow. To perform these functions there are at least 20 different types of neuron including excitatory motor neurons, inhibitory motor neurons, interneurons, sensory neurons, and secretomotor neurons. Collectively, these neurons express every neurotransmitter found in the brain. There are several excellent recent reviews about ENS development including (we hope) one that we wrote.

We are interested in determining the molecular mechanisms that control enteric nervous system development. This is important because defects in ENS development cause serious problems with intestinal function. This includes Hirschsprung disease, intestinal pseudo-obstruction syndrome and probably some forms of irritable bowel syndrome. Symptoms may include constipation, vomiting, and growth failure. Some of the more severe forms of intestinal motility disorder may be fatal. By defining the molecules that are critical for ENS development, we hope to find new ways to prevent and treat intestinal motility disorders.

Projects:

1. Retinoids in ENS development: We have shown that retinoids (vitamin A metabolites) are critical for ENS precursor migration into the distal bowel and that retinoids influence ENS precursor proliferation and differentiation. In part the effect of retinoids in the developing ENS is mediated via retinoid acid induced reductions in PTEN protein in the most actively migrating ENS precursors. Our recent analysis of RALDH mutant mice suggests complex roles for retinoids within the ENS and within other cells of the bowel wall. We have a lot more to learn about how and why retinoic acid signaling affects ENS development. More details about our work are found at (Heuckeroth ENS/Retinoids)

This work has interesting implications for human birth defects including Hirschsprung disease and other intestinal motility disorders. In particular, we now hypothesize that vitamin A deficiency may increase the risk for Hirschsprung disease at least in regions where vitamin A deficiency is common. Formally testing this hypothesis will require international collaborators since vitamin A deficiency is rare in the United States. We would welcome this type of collaborative work. As a note of caution, too much vitamin A also causes birth defects (including ENS defects), so high dose supplementation during pregnancy needs to be avoided.

Ongoing studies are focused on determining the relative importance or retinoic acid signaling within ENS precursors or in other cells of the bowel wall. Defining the signaling pathways that cause the profound effects of retinoids on the developing ENS is already leading to new insight into how the ENS forms and we are looking for students to pursue this work in more detail.

2. Gene-environment interactions and birth defect prevention: After more than a decade studying signaling pathways that control ENS development, we recognized that many of the molecules we were investigating are influenced by non-genetic factors. Recent work led to the identification of several medicines that affect ENS development. Ongoing studies examine how some of these medicines affect the developing ENS and we are interested in extending this work to humans. These studies suggest that some human intestinal motility disorders, including Hirschsprung disease may be preventable by avoiding specific medicines during early pregnancy.

3. Signaling pathways in ENS development: Development of the ENS requires coordinated cell migration and proliferation of precursors followed by differentiation into neurons and glia. This process is guided and supported by a wide array of molecules. There are well established roles for tyrosine kinase receptors and co-receptors (RET, GFRalpha1, GFRalpha1, TRKC) and their ligands (GDNF, NRTN, NT3), the G-protein coupled receptors (EDNRB), the ligand EDN3, and protease ECE1, morphogens and differentiation factors (BMP2/4, SHH, IHH, RA), transcription factors (SOX10, ZFHX1B, PHOX2B), and many additional intracellular, extracellular and transmembrane proteins (GSK3, PKCzeta, RAC, RHO, SMURF1, KBP, laminin, fibronectin, etc.). These proteins have additional roles during development and in post-natal life. Because diverse cell types respond differently to the same protein, essentially all of our studies are performed in vitro or in vivo using ENS precursors and surrounding cells.

4. Ret Signaling in ENS Development:Ret is a growth factor receptor that transduces signals from four ligands (GDNF, neurturin, artemin and persephin) via four glycosylphosphatidylinositol linked co-receptors (GFRalpha1, GFRalpha2, GFRalpha3 and GFRalpha4 respectively). Inactivating Ret mutations occur on one chromosome in 50% of people with familial Hirschsprung disease and in 15% of people "sporadic" disease. However, most people with Hirschsprung disease have either an inactivating Ret mutation or changes in their DNA that reduce Ret expression. We have been studying Ret since 1994, but many investigators around the world also study this protein. Our work with Ret signaling led to our interest in the ENS.

What have we learned about Ret? Our ongoing work has demonstrated that Ret signaling is important for ENS precursor survival, proliferation, migration, neuronal differentiation and neurite growth. These diverse roles for Ret explain why inactivating Ret mutations predispose to Hirschsprung disease. Less severe Ret mutations may also cause defective intestinal motility without causing aganglionosis. We have demonstrated a wide array of structural and functional defects in mutant mice. More details about our work on Ret, links to our manuscripts, and a few links to the work of other investigators are found at (Heuckeroth Ret research).

5. Finding New Genes Important for ENS Development: Ongoing studies suggest that hundreds of molecules influence ENS development and function. Given all of the features of the ENS that must be specified and directed, it is likely that we still only know a fraction of the molecules required to make a functioning ENS. For example, very little is know about the molecular mechanisms that determine what type of neuron an ENS precursor will become. Mechanisms that guide enteric neuron axons or that influence regional patterns of intestinal innervation remain poorly understood. Mechanisms that cause adjacent cells to adopt different fates are essentially unknown. To approach these problems it is essential to know more about patterns of gene expression in the developing bowel.

We have used a wide variety of approaches to identify genes expressed within the ENS and in cells surrounding developing ENS precursors. Using these methods we have identified many "candidate" molecules that may influence ENS development and functional roles for many of these proteins. More details about our work to identify new molecules that influence ENS development is at: (Heuckeroth New ENS genes research).